1. Field of the Invention
The present invention relates to the field of hearing aids. The invention more specifically, relates to hearing aids utilizing gain-limitation. The invention, more particularly relates to hearing aids having means for estimating the acoustic loop gain and, still more particularly, relates to hearing aids further incorporating gain limitation in order to reduce disturbances due to acoustic feedback, and respective systems and methods thereof. In addition the invention relates to a system exploiting the increase in gain margin due to the utilization of feedback cancellation techniques by permitting larger signal path gain in the hearing aid.
2. Description of the Related Art
It is a general object in hearing aid design to establish the maximum possible amount of gain with which an acoustic input signal may be amplified to produce a hearing loss compensation signal without the appearance of acoustic feedback or other acoustic disturbances.
WO-A-94/09604 discloses a hearing aid with digital, electronic compensation for acoustic feedback, which comprises a compensation circuit. The circuit monitors the loop gain and regulates the hearing aid amplification so that the loop gain is less than a constant K. An adaptive filter operates to minimize the correlation between input and output from the hearing aid and may be used to give a measure of the attenuation in the acoustic feedback path by deriving gain and phase characteristics from a feedback cancellation filter.
WO-A-02/25996 discloses a hearing aid with an adaptive filter for suppression of acoustic feedback. The adaptive filter may be used as an independent measuring system to estimate the acoustic feedback signal without distortion of the processed acoustic input signal.
Data derived from the adaptive filter may be used to determine loop gain, which may be utilized to set an upper limit on the applicable gain that may be used in each of multiple evaluated frequency bands.
It is further known that a large autocorrelation measurement may indicate feedback oscillation. Accordingly, feedback detectors that rely on autocorrelation measurements have been suggested in the prior art.
However, neither of these documents discloses how in situations with high, and increasing, autocorrelation, a gain limit could be identified in situations where the known solutions, e.g. measuring gain in the acoustic feedback path with an adaptive feedback cancellation filter, cannot be relied upon.
The most common technique to alleviate feedback oscillations is gain-reduction. Managing feedback by gain reduction is in particular a problem in linear hearing aids. Most linear hearing aids are adapted for greater gain in the high frequencies, where the hearing deficiency tends to be more profound. Unfortunately, the typical feedback path also provides less attenuation at high frequencies than at low frequencies. Therefore, the risk of audible feedback is highest in the higher frequency range. One common method to control feedback is to lower the high frequency gain of the hearing aid. However, speech intelligibility may suffer as a consequence.
Therefore, disturbances in the output signal of a hearing aid as well as instability and limited available gain are still challenges in today's hearing aid design.
Thus, there is a need for improved hearing aids as well as improved techniques for utilizing gain-limitation in hearing aids.